Image forming apparatuses such as a laser printer, LED (light emitting diode) printer, or a digital photocopying machine, employ an electrophotographic method. These apparatuses electrify the entire surface of a photoreceptor and create a desired electrostatic latent image by irradiating the surface with light in accordance with image information by use of a laser beam, LED or the like. The electrostatic latent image is visualized with a developer by a developing section to form a visible image, and the visible image is fixed onto a recording medium, thereby obtaining an image.
Recently, there has been an increasing demand for size-reduced image forming apparatuses. In an image forming apparatus employing the electrophotographic method, the developer storage section is a significant part of the image forming apparatus in terms of achieving size reduction because the space occupied by the developer storage section is large. In particular, in the recent networked environment, a single image forming apparatus is used by a number of people for a great number of printouts. Therefore, for users' convenience, a large quantity of developer must be stored in the image forming apparatus.
Recently, there has also been an increasing demand for color image output. Since a color image forming apparatus uses developers of three or four colors, the space occupied by the developers in the image forming apparatus becomes much larger. Further, when color reproduction for color images is carried out by multi-color overlapping, the amount of developer used on a recording medium (e.g., paper and an OHP sheet) becomes large. For thermally fixing such large amounts of developer, larger amounts of heat are necessary compared with the case of monochrome images, thus requiring a large fixing section.
In addition, there is a demand for a method for manufacturing a developer that saves more energy and is more environment-friendly. Developer manufacturing methods that are currently available and mainly used include a conventional method involving melting, kneading, and grinding processes, and recently introduced polymerization methods (e.g., suspension method, emulsion method, and dispersion method) in a liquid solvent.
For example, a developer used for a dry developing method contains a thermoplastic resin (binding resin), a pigment (coloring agent) and a mold release agent as main components. In addition, magnetic powder, a charge control agent, a flow improver or the like may be added, if necessary, for the production of the developer. Then, a method for manufacturing such developer is generally adopted wherein all the raw materials are mixed at one time, and heated, melted and dispersed by a kneading machine, etc. to obtain homogeneous composition. Then the obtained composition is cooled, crushed, and classified to obtain a developer having a volume average particle diameter of approximately 10 μm.
In particular, a color developer for electrophotography used for the formation of color images is generally manufactured by dispersing various chromatic color pigments in a binding resin. For such cases, performances required for the developer that is used are higher than those in the case of obtaining black images. In other words, as a developer, proper color development (color degree) or optical transparency (transparency), when the developer is used for a sheet for an overhead projector (OHP), is required, in addition to mechanical and electronic stability with respect to extrinsic factors such as mechanical shocks or humidity.
A developer that contains a pigment as a coloring agent is exemplified by JP Patent Publication (Kokai) No. 49-46951 A (1974) stated below. However, a pigment-containing color developer exhibits poor pigment dispersibility in a binding resin, resulting in inferior color degree (color development) and transparency, although it has excellent light resistance.
As methods for improving the dispersibility of a pigment in a binding resin, the following technologies are proposed.    (1) A technology for obtaining a color developer by using a polyester resin (resin A) as a binding resin, coating a pigment in advance with a polyester resin (resin B) with a molecular weight higher than resin A, and dispersing the coated pigment in resin A (JP Patent Publication (Kokai) No. 62-280755 A (1987)).    (2) A color developer characterized in that a processed pigment obtained by melting and kneading a pigment and a resin for the pigment is dispersed and contained in a binding resin, the resin for pigment has a smaller weight average molecular weight than the binding resin, and the binding resin has a weight average molecular weight of 100,000 or more (JP Patent Publication (Kokai) No. 2-66561 A (1990)).    (3) A technology for obtaining a color developer by the following steps. In a first step, a mixture of a binding resin and a pigment is kneaded with an organic solvent at a temperature lower than the melting point of the binding resin. In a second step, the binding resin and a charge control agent are further added, and melted and kneaded by heat (JP Patent Publication (Kokai) No. 9-101632 A (1997)).    (4) A pigment used for a developer has a low molecular weight material absorbed therein, the low molecular weight material having a lower melting point and a lower melt viscosity than a binding resin as a principal constituent component of the developer. The oil absorption of the low molecular weight material is 50 g or more (per 100 g of pigment) and the oil absorption ratio of the low molecular weight material to the pigment is 100% to 300% of the saturation oil absorption. The melt viscosity of the low molecular weight material is 0.1 Pa·s or less at 20° C. (the melting point of the low molecular weight material). In addition, a pretreatment method of developer pigment, a developer and a manufacturing method of the developer are also proposed in JP Patent Publication (Kokai) No. 2000-81736 A.
However, none of the methods disclosed in these patent documents can provide sufficient pigment dispersion, thus currently resulting in inferior color degree and transparency. Further, in the case of a black developer for monochrome images, carbon is commonly used as a black coloring agent in an amount of 7 to 15 parts by weight. For manufacturing such black developer, a method is commonly used wherein carbon powder is mixed with other raw materials before kneading and then the mixture is melted and kneaded. Unlike a color developer, transparency is not required for a black developer, and therefore the amount of carbon is increased to enhance color degree. However, the increase of conductive carbon is not preferable in terms of the stability of electrical charge because it reduces the volume resistivity of the developer. It is thus necessary that the carbon should be thoroughly dispersed to increase the volume resistivity of a developer.
A two-step kneading method for the above color developer has not been adopted as a method for improving carbon dispersion due to its high cost, and in general (5) a method for reducing the treatment capacity in the kneading step is adopted. In addition, (6) a method for lowering resin temperature in the kneading step or (7) a method in which a rolling and cooling method is prescribed after kneading is proposed. However, the treatment capacity according to methods (5) to (7) is small, resulting in increased costs.
JP Patent Publication (Kokai) No. 8-141306 A (1996) discloses a technique regarding a method and a device for extraction of a fluid flowing in a radial direction whereby a predetermined material is extracted from a solid material (e.g., naturally occurring material and ceramics) using a supercritical fluid or a liquid. However, the technique disclosed therein is insufficient for reducing extraction period, and further, it is expensive since the treatment capacity is small.